The present invention relates to a new method, based on laser technology, of initiating explosive charges, and a device which is intended for initiating explosives and in accordance with said method functions according to entirely new principles. The basic idea underlying the invention is to ignite the explosive charge concerned not as previously proposed by means of the radiation emitted from a laser but by way of self-destruction or overheating of a laser source assembled together with the explosive charge. In this regard, the aim is to cause the laser source to melt down or explode and, in connection with this, to initiate the explosive. With the present invention, it has suddenly become possible to use even very small laser sources of the mini or micro type for triggering explosive charges where it was previously necessary to use very powerful laser sources for the same purpose.
Although there are a number of different ignition systems based on pyrotechnics for explosive charges, most conventional ignition systems intended for this purpose are based on electric ignition. This is true of both civil and military applications. The common disadvantage of all electric ignition systems is their great sensitivity to external influences, which makes them difficult to handle in a completely safe manner because this sensitivity is difficult to design out. The problem is accentuated on account of the fact that in modern society we are surrounded by more and more radiofrequency radiation, at the same time as the electric conductors which are unavoidable in electric igniters can always function as antennas, which can give rise to accidental triggering.
The optical maser or laser (Light Amplification by Stimulated Emission of Radiation) exists in a countless number of forms depending mainly on the laser material used. However, the basic principle of all laser types is that amplification of light is brought about by stimulated emission. For this purpose, in the first place a suitable laser material is required, in which the relevant components may be atoms, molecules or electrons, with at least two well-defined energy states. If gas lasers are disregarded, the laser material normally consists of a preferably rod-shaped crystal material, for which reason reference is often made to laser crystals or laser rods. Also required for the functioning of the laser is an energy source which supplies energy to the laser material in a quantity and form which can excite the active components in the laser material to a higher energy state, which means that the material begins to lase, that is to say to transmit a laser beam. Supplying energy to a laser is usually referred to as pumping the laser, and this can be effected in many laser materials by supplying light, which is also preferable in connection with the present invention. The third essential component of the laser is an optical resonator in the form of at least two mirrors arranged at the ends of the laser crystal and oriented in such a manner that the radiation inside the crystal is reflected between the mirrors. When the aim is to take a laser beam out of the laser source, one of the mirrors must be semi-transparent so that part of the radiation which bounces between the two mirrors of the resonator can come out. The generation of the laser beam itself begins with photons spontaneously emitted in all directions from the pumped laser material, and the photons which are reflected on the resonator mirrors are returned into the laser material and there cause stimulated emission of photons with the same wavelength, direction and phase. It is these properties which give the laser beam its coherent properties. In a conventional laser, part of the radiation is then taken out via the semi-transparent mirror. As long as the laser is pumped with energy, the laser beam will continue to be emitted.
In addition to the lasers which use solid and then preferably crystalline laser material, there are also, as already indicated, gas lasers, and amongst these there are also lasers consisting of specific gas mixtures which can be pumped with light and therefore could be of interest in connection with the present invention. However, those lasers which have to be pumped with electrical energy are of less interest in this context because these, owing to the fact that they require electric conductors for supplying the pumping energy, in principle have the same weaknesses in terms of safety as conventional electric igniters.
Over a number of years, the space and military industry has developed and made use of laser-based ignition systems. In such laser-based ignition systems, the laser is utilized in order to generate a heat pulse which is supplied to the ignition unit via a fibre optic light conductor or cable. These laser igniters have nevertheless proved very expensive because they require very powerful and thus expensive laser sources even when special amplification elements, for example lenses or convex mirrors, are used between the laser source and the initiation location. Laser-based explosive igniters have therefore hitherto been used principally in more exclusive technical areas where the price has not been too crucial a factor.
The advantages of a laser-based ignition system are primarily associated with its great safety in that it can be shielded from every form of external influence.
The theoretically simplest laser igniter for an explosive charge is that which quite simply consists of a fibre optic light conductor of which the outer end is coated with a conventional pyrotechnic composition which will therefore be ignited by the heat generated by a laser beam sent through the optical cable. This variant is simple and reliable but requires a very powerful laser source at the other end of the optical cable.
A slightly weaker laser source can be used if the laser beam is amplified directly before the pyrotechnic composition, and this can be effected by, for example, an optical lens, optical mirrors or a fibre optic light amplifier. All these previously proposed solutions are practicable, but the necessary laser source is of not inconsiderable strength in these variants as well and thus still relatively expensive.
In a further variant which manages with a somewhat weaker laser source, an IR-absorbing material is arranged between the outer end of the optical cable and the pyrotechnic composition at the same time as the heat absorption capacity of the latter is augmented by, for example, adding carbon powder. The laser intended for igniting a pyrotechnic composition can also, by way of the selection of laser-emitting material, be tailored to the optimum absorption wavelength of the pyrotechnic composition used. Even if this is done, relatively strong lasers are nevertheless still required in order to ignite a pyrotechnic composition by laser in accordance with the previously known art described very briefly above.
A further variant which has the special effect that it provides exploding ignition but which requires a very powerful laser source is the laser igniter which starts by gasifying a suitable medium, for example a plastic film, and accelerating this medium through a tube towards the explosive to be initiated.
Another known basic principle for laser igniters for explosives is characterized in that a laser diode is arranged in direct proximity to or inside the explosive and in that this laser diode is supplied with electric voltage when the explosive is to be initiated. In precisely the same way as a conventional electric igniter, however, this igniter is dependent on an ignition current which is supplied via ordinary electric conductors and it is therefore affected just as easily by electromagnetic pulses from other electrical equipment as the conventional electric igniters and can therefore be used to the same limited extent as these, without extra safety arrangements, in situations where other electrical equipment may be used in the immediate surroundings.